Radio frequency or microwave (hereinafter “RF”) plasma generation equipment is widely used in semiconductor and industrial plasma processing. Plasma processing supports a wide variety of applications, including etching of materials from substrates, deposition of materials onto substrates, cleaning of substrate surfaces, and modification of substrate surfaces. The frequency and power levels employed vary widely, from about 10 kHz to 10 GHz, and from a few Watts to as much as 100 kW or greater. For semiconductor processing applications, the range of frequencies and powers presently used in plasma processing equipment is somewhat narrower, ranging from about 10 KHz to 2.45 GHz and 10 W to 30 kW, respectively.
Plasma processing equipment typically requires a precision RF signal generator, a matching network, cabling, and metrology equipment. In addition, precision instrumentation is usually required to control the actual power reaching the plasma. The impedance of loads associated with a plasma can vary considerably in response to variations in the gas recipe, various gas supply parameters, plasma density, delivered RF power, pressure and other variables. The dynamic electrical load presented by the varying impedance of a plasma load, and these other variables, can create significant stability control problems for plasma generation equipment.
In today's plasma processing equipment, such as semiconductor tools, it is often observed that the process becomes unstable, exhibiting oscillations in plasma density and/or loss of plasma altogether. Thus, a need exists to prevent these instabilities by changing the way RF power supply is regulated.
A known plasma generation system including a typical plasma processing tool and an RF power supply, familiar to those skilled in the art, is depicted in FIG. 1. An RF power supply 10 delivers RF power through a filter 12 and to the process vacuum chamber 15 via cables 13 and a load matching circuit 14. RF power, both forward and reflected, is measured at output power measurement point 18, near where the power enters the process vacuum chamber 15. Feedback from the output power measurement 18 is sent back to the power supply 10, to complete the control loop.
Power regulation control circuitry is generally designed to operate as fast as possible, to maximize control stability. A typical power supply control loop can operate as fast as a few hundreds of microseconds, and is usually designed and optimized in view of a fixed load, or perhaps with a range of possible fixed loads in mind. The speed of the control loop, e.g., in a plasma generation system, is limited by filter and matching network delays, power measurement time and the internal reaction speed of the power supply. As described above, the plasma introduces additional delays in the control loop that may ultimately result in unstable system behavior. These delays are not easily predictable, are process dependent, and are inconsistent.
However, many power regulation control instabilities can be attributed to having the plasma as a part of the power regulation control loop, since plasma impedance is not constant. More specifically, plasma impedance is a function of the amount of power delivered, the plasma gas pressure, and the chemical composition of the plasma gas mixture. Moreover, the reaction chemistry and gas pressure variables are dependent upon plasma density and temperature, and also have inertia (latent time dependencies) of their own. These inertia times can be in the range of from microseconds to milliseconds, and are determined by various processes, such as diffusion rates, pumping speed, ionization and chemical reaction rates, etc.
Another complicating factor is introduced by the gas supply system 20, which oftentimes has a control loop of its own. This gas supply control loop 22 commonly controls the gas supply to the chamber 15 based on a chamber pressure measurement 23. This separate control loop 22 can extend the specific and system inertia (delay) times of the power control system to hundreds and thousand of milliseconds. Thus, any power supply that uses only a feedback control loop 25 with a response time interval that is slower than or equal to the switching frequency of the system or plasma inertia, i.e., the response time of the electrical load (such as the plasma), is potentially prone to instability.
FIG. 2 illustrates the power section of a known power supply technology that can be used with a system such as the plasma processing system illustrated in FIG. 1. A power supply 30 (e.g., a DC or RF switching power supply) is designed to deliver a constant power output value to a load 40 at the output of the power supply. DC power 32, e.g., from a rectified bus or a DC power source, feeds switching transistors and reactive elements such as inductors and capacitors, as is known to those of skill in the art. Power supply 30 generally delivers constant current or constant voltage on a time scale (i.e., at a time interval) faster than the response time of its control loop 33. The control circuit 35 adjusts a control signal to the power supply 30 based on information received from the feedback control loop 33. Of course, power supply 30 can also deliver constant current or power when the control loop is frozen (open). For reference, such power supplies are referred to as “constant current” or “constant voltage” types, below. The control circuit 35 uses feedback 33 from the plasma power measurement circuit 18 to modify the output voltage or current of the power supply 30, to maintain the desired power output. In some situations this can result in stable control of the load 40 (e.g., a plasma load).
In practice, one example of known technology is a “constant current” DC power supply which is used to power an arc-type plasma load. Since plasma resistance drops at higher temperatures (often referred to as “negative resistance”), it is known that supplying a constant current to such a system maintains a stable plasma. In such systems, the power supply functions somewhat as a virtual ballast resistor.
More commonly, many DC switching power supplies use a pulse width modulation (“PWM”) control method. These behave as voltage sources and thus are unable to effectively sustain a negative-resistance plasma without a feedback control loop. Moreover, when plasma systems are powered with an RF power supply control problems are exacerbated, since the phase of the reflected power is shifted by cables (e.g., 13) and filters (e.g., 12) to such an extent that it is difficult or impossible to ensure either positive or negative apparent plasma resistance, as viewed from the power supply. Such problems are especially pronounced for systems operating at high frequencies, e.g., in the RF region (i.e., in the range of 1 MHz and up).